The invention relates to a method and device for expanding metal tubes, particularly metal tubes of oval cross-section, to fasten the metal tubes to metal ribs.
It is known from EP-B1 188 314 to connect metal tubes of oval or elliptical cross-section and metal ribs to one another by mechanically expanding the metal tubes. Here, expansion elements are pushed through the metal tubes, the expansion operation taking place in two stages.
A similar single-stage expansion method and a device in the form of an expansion mandrel having an expansion head and a thrust rod is disclosed by DE-A-27 05 632.
A disadvantage in the above-mentioned prior art is that the metal tubes are under compressive load during the expansion and can consequently bend out or buckle. Leakages can occur at the tube-plate connection due to this bending or buckling of the tubes. The mandrel rod is likewise under compressive load. Here, too, there is a risk of buckling, the expansion element tilting in the metal tube and the expansion of the tube becoming uneven.
The present invention also relates to an apparatus for mechanically expanding metallic tubes, in particular tubes in heat exchangers, the apparatus including a drawing mandrel with an expanding element and a holding device. It is suitable, for example, for a process for producing heat exchangers, such as that described in the first part of the present invention. For each tube to be expanded, such an apparatus has a drawing mandrel with an expanding element and at least one holding device. The tubes are pre-expanded at their ends and fitted into the holding device. Subsequently, the tube is passed through by the drawing mandrel and the expanding element, the tube being expanded. The pre-expanding is necessary in order that the expanding element can be passed completely through the tube and does not jam in the holding device. The pre-expanding of the tubes is an independent operation and consequently a not inconsiderable cost factor.